Add trackpad pinch-to-zoom gesture to the comic viewer

[codeKonami]

Relates to #435.

What this adds

Native trackpad pinch-to-zoom in the comic viewer:

• Continuous zoom from 30% to 500% (the existing bounds used by Cmd +/Cmd -) driven by the trackpad pinch gesture.
• Cursor-anchored zoom. The viewport is scrolled so the point under the cursor at the start of the gesture stays under the cursor while scaling, rather than zooming around the viewport center.
• Compact bottom-right HUD showing the live percentage while the gesture is active, so it does not obscure the page being read. The existing centered notification is preserved for keyboard Cmd + / Cmd - zoom.
• Preferences toggle. A new Zoom mode group is added to Preferences > General, below Mouse mode, with a single checkbox to enable or disable the gesture. Defaults to enabled.

How it works

• Viewer now grabs Qt::PinchGesture in its constructor and overrides event() to dispatch QEvent::Gesture to a new gestureEvent(QGestureEvent *).
• The gesture handler captures the zoom value and the cursor position in content coordinates at Qt::GestureStarted, then on Qt::GestureUpdated applies baselineZoom * pinch->totalScaleFactor() clamped to [30, 500], resizes the content, and adjusts the horizontal/vertical scroll bars to keep the anchor stationary.
• The HUD is a dedicated QLabel child of the viewer, rendered as Qt rich text to keep the "%" text reliably white over the translucent background regardless of palette cascade. It shows on GestureStarted and hides on GestureFinished / GestureCanceled.
• The setting is persisted as PINCH_TO_ZOOM_ENABLED in the existing QSettings file, exposed via Configuration::getPinchToZoomEnabled() / setPinchToZoomEnabled(), and checked on each gesture event so toggling in Preferences takes effect immediately.

Cross-platform

No #ifdef Q_OS_MACOS is needed. QPinchGesture is handled natively by Qt on macOS (trackpad) and Windows (touch), and is a no-op on platforms that do not deliver pinch events, so the feature is additive and does not regress existing behavior on Linux or on systems without a touch surface.

Tested

• Built with Qt 6.11 on macOS 15 (Apple Silicon).
• Verified: gesture zooms and dezooms smoothly, cursor-anchored point stays under the cursor, Cmd + / Cmd - shortcuts and their HUD still work, two-finger scroll and the magnifying glass still work, the Zoom mode checkbox enables/disables the gesture at runtime.
• clang-format passes on the modified files.
• No change to existing public API, and no new runtime dependency.

Both Qt 5 and Qt 6 code paths should compile (QPinchGesture and QGestureEvent are available in both), though I have only been able to test Qt 6 locally.